Annoying sound when 'sound on' is toggled from silent

Apologies for the awful thread title.

When I toggle between silent/vibrate/sound on from the pull down menu on on my S6 Edge I get an annoying sound when I enter 'sound on' mode.

I'm experienced in deleting annoying sounds in the UI file (I'm rooted) but I can't work pout which sound file I need to change/delete.

Anyone know which one it is please?